So called preforms are used for the production of plastic bottles. The preforms are made of a thermoplastic material, especially PET. The preforms are first heated and then reshaped into the desired container shape in a stretch blow device. Usually the preforms are conveyed through a heating device on a conveyor path before being processed in the stretch blow device.
Each preform is already designed with an upwardly open mouth region, which usually comprises an external thread. This mouth region essentially corresponds to the mouth region of the finished bottle. The preforms are typically injection molded parts. The preforms are tempered to the required forming temperature in a heating device. The tempered preforms are then processed and reshaped into the final container or bottle shape in a stretch blow device.
The heaters for tempering the preforms via infrared radiation are arranged in heating boxes. The heating boxes are arranged on the outboard side of the straight long sides of the heating device. Usually reflectors are arranged on the inside and on the floor part of the heating boxes. Thereby the emitted infrared radiation can be used as much as possible because the infrared radiation is advantageously deflected in the direction of the preforms.
In most known systems the mouth region of the preforms is protected from excessive heating by an air stream. This is necessary because otherwise the mouth region might be deformed in the subsequent blow molding process. This would lead to the production of incorrect containers that need to be discarded.
Furthermore, the surface of the preforms needs to be protected from burning which might occur due to excessive local heating. This is usually done by injecting air into the heating system. The air is usually injected through slits in the reflectors that are arranged opposite to the heating elements.
A schematic representation of a heating device 1 for tempering preforms 20 according to the known state of the art is shown in FIG. 1. The preforms 20 are delivered by transport means or transporter 2, then divided and spaced as required by a sawtooth starwheel 3a and fed into an oven or heating module 4, where they are moved along a heating tunnel 5 in a transport direction F.
While moving through the oven 4, the preforms 20 first pass through a first linear heating zone 5a, whereby their circumferential surface is heated uniformly. After a deflection zone 6 at one end of the oven 4, the movement of the preforms 20 continues through a second linear heating zone 5b. In the second linear heating zone 5b the preforms 20 are tempered to the final temperature before being processed into the final shape in a stretch blow molding device. Multiple heating boxes 7 are arranged in the linear zones 5a, 5b of the heating tunnel 5. The heating boxes 7 comprise infrared emitters and additional reflectors.
The reflectors are required in order to minimize radiation losses as much as possible. The radiation emitted by the infrared emitter, which does not enter the preforms 20, is reflected back to the preforms 20 by the reflectors and therefore not lost.
While the preforms 20 pass through the oven 4, they are preferably rotated to ensure a uniform heating of all sides. Then the heated preforms 20 are passed, for example, to an outlet starwheel 3b and fed into a stretch blow molding device or something alike.
FIG. 2 shows a heating box 7 known from prior art for use in a heating device according to FIG. 1. FIG. 2 especially shows a heating alley 8 with a heating box 7 used in one of the linear heating zones 5a, 5b in the heating module 4, 5 (see FIG. 1). The preform 20 is mounted on holding means or holder 16 and preferably rotated during its movement through the heating alley 8. To prevent excessive heating of the mouth region 22 of the preform 20, the mouth region 22 is shielded from direct radiation by a shielding plate 9. Furthermore an outer cooling plate 10 is arranged in this region. This outer cooling plate 10 shields the mouth region 22 from upward radiation.
In a heating alley 8 infrared emitters are arranged in a vertical row parallel to the longitudinal axis X of the preform 20. Therefore the preform 20 gets irradiated and heated over its entire height. The term parallel refers to a plane, in which the majority of the radiant emitters 11 are arranged. The longitudinal axis of each rod-shaped radiant emitter 11 is perpendicular to the longitudinal axis X of the preform 20. The electrical power of each radiant emitter 11 can be controlled separately. Thereby a temperature profile can be applied to the preform 20 along its longitudinal axis X. All radiant emitters 11 are arranged in the same distance from the longitudinal axis X of the preform 20. Furthermore, a bottom radiant emitter 12 is arranged in the lower part of the heating alley 8, which irradiates and thereby heats the preform 20 from below.
To ensure the best use of the radiation from the radiant emitters 11, 12, the heating alley 8 comprises reflectors, especially a counter reflector 13, a bottom reflector 14 and a back reflector 15. The counter reflector 13 is arranged behind the preform 20, opposite to the radiant emitters 11. The back reflector 15 is arranged behind the radiant emitters 11 thus reflecting the radiation that could otherwise not be used for the heating of the preforms 20.
It is necessary to have a certain minimal distance d1 between the back reflector 15 and the infrared radiant emitters 11 to prevent damage of the back reflector 15 due to the high temperature fluctuations in the heating alley 8.
EP1 278 619 B1 describes a heating assembly with elongated heaters. A single reflector is associated with each heater. The reflector has a parabolic shape, therefore the reflected radiation beams are generally parallel to each other which ensures a controlled tempering of the preforms.
DE 100 58 950 B4 shows a heating route for preforms with elongated halogen light sources. The reflectors are arranged around the preform and form a substantially closed radiation space.